Are you running a regulator? The 7955 isn't a high voltage servo, so if your running a LiPo to your reciever the servos are getting 7.4 volts as compared to the 6.0 volts max that the servo's require can cause the servo to do that and damage it at the same time. You can also check the end point adjustments (EPA) and adjust the distance the servo travels back and forth on the throttle.